Nonyellowing cathodic electrocoat

- BASF Corporation

A method of preventing the yellowing of the outermost coating of a multicoat coating system is disclosed. The method comprises initially depositing onto a conductive substrate by cathodic electrodeposition a primer coating comprising at least one layer of an amine-epoxy resin adduct and a cross-linking agent, and then curing said primer to a hard, durable film. Then, depositing a second coating onto the primer layer comprising at least one layer of a pigmented film-forming basecoat. Then depositing a third outermost coating onto the second coating comprising at least one layer of a clear, film-forming topcoat, and then curing the basecoat and topcoat. The primer coating contains a cross-linking agent selected from the group consisting of aliphatic polyisocyanates containing at least six carbon atoms, the isocyanurates of aliphatic polyisocyanates containing at least six carbon atoms, aromatic polyisocyanates having a molecular weight greater than 174, and the isocyanurates of aromatic diisocyanates having a molecular weight greater than 174. The polyisocyanates or isocyanurates are blocked with a blocking agent.

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Description
EXAMPLE 1A

The following components were charged into a suitable reactor vessel:

1658 parts of Epon 828 having an epoxy equivalent weight of 188;

473 parts of Bisphenol A;

583 parts of ethoxylated Bisphenol A having a hydroxy equivalent weight of 230 (Synfac 8009 from Milliken Chemical Co.); and

147 parts of toluene.

The charge was heated to 145.degree. C. under a dry nitrogen blanket and 3.8 parts benzyl dimethyl amine were then added to the reactor vessel. The reaction or mixture was further heated to 160.degree. C., and held for 45 minutes. An additional 5.4 parts of benzyl dimethyl amine was added, and the mixture was held at 150.degree. C. until the desired WPE (weight per epoxide) was achieved, then 1037 parts of toluene were added to the reactor to dilute the resulting epoxy adduct.

A conventional reactor equipped with an agitator, thermometer, nitrogen line and a condenser was charged with 900 parts of diethylene triamine. The diethylene triamine was slowly heated to 140.degree. F. Then, 2869 parts of the epoxy adduct were slowly added to the reactor during a one-hour time period. After the epoxy adduct was completely charged, the mixture was heated to 180.degree. F. and held for one hour. Next, the excess amine in the reactor mixture was vacuum distilled, condensed and removed by applying a vacuum of 75 mmHg and slowly raising the temperature of the reactor charge to 550.degree. F. over a 2.5 hour time period. The mixture was held at this temperature until no more distillate was observed to be coming out. The temperature was then lowered to 360.degree. F. and 283 parts of Pelargonic acid along with 200 parts of xylene were added to the reactor. The resulting mixture was heated to 360.degree. F. and held at reflux until the acid value was down to 6. Then the reaction mixture was cooled down to ambient temperature and reduced to 71.4% with NV methyl isobutyl ketone.

EXAMPLE 1B

An ethylene glycol monopropyl ether blocked isocyanate cross-linker was prepared by slowly charging 870 parts of trimethylopropane into a suitable reactor vessel containing 3387 parts of an 80/20 isomer mixture of 2,4-/2,6-toluene diisocyanate (TDI), 1469 parts of methyl isobutyl ketone, and 2 parts of dibutyl tin dilaurate under agitation with a nitrogen blanket. The reaction was maintained at a temperature below 110.degree. F. The charge was held an additional one and one-half hours at 110.degree. F. and then heated to 140.degree. F. at which time 2026 parts of ethylene glycol monopropyl ether were added. The charge was maintained at 210.degree. F. to 220.degree. F. for one and one-half hours until essentially all of the isocyanate moiety was consumed as indicated by infrared scan. The batch was then thinned with methyl isobutyl ketone to 74% N.V.

EXAMPLE 1C

An ethylene glycol monohexyl ether blocked isocyanate cross-linker was prepared slowly charging 870 parts of trimethylopropane into a suitable reactor vessel containing 3387 parts of an 80/20 isomer mixture of 2,4-/2,6-toluene diisocyanate, 1469 parts of methyl isobutyl ketone, and 2 parts of dibutyl tin dilaurate under agitation with a nitrogen blanket. The reaction was maintained at a temperature below 110.degree. F. The charge was held an additional one and one-half hours at 110.degree. F. and then heated to 140.degree. F. at which time 2844 parts of ethylene glycol monohexyl ether were added. The charge was maintained at 210.degree. F. to 220.degree. F. for one and one-half hours until essentially all of the isocyanate moiety was consumed as indicated by infrared scan. The batch was then thinned with methyl isobutyl ketone to 74% N.V.

EXAMPLE 1D

An aliphatic caprolactam blocked isocyanate cross-linker was prepared by slowly charging a solution of 1680 parts of caprolactam, 665 parts of trimethylopropane, and 2467 parts of toluene into a suitable reactor vessel containing 3307 parts of isophorone diisocyanate (IPDI). The charge was held an additional hour at 150.degree. F. Then, 14 parts of dibutyl tin dilaurate were added, and the charge was heated to 205.degree. F. The charge was maintained at 210.degree. F. to 220.degree. F. for about one hour until essentially all of the isocyanate moiety was consumed as indicated by infrared scan. The batch was then thinned with methyl isobutyl ketone to 74% N.V.

EXAMPLE 1E

A caprolactam blocked aromatic isocyanate cross-linker was prepared by slowly charging 870 parts of trimethylopropane ito a suitable reactor vessel containing 3387 parts of an 80/20 isomer mixture of 2,4-/2,6-toluene diisocyanate, 1469 parts of methyl isobutyl ketone, and 2 parts of dibutyl tin dilaurate under agitation with a nitrogen blanket. The reaction was maintained at a temperature below 110.degree. F. The charge was held an additional one and one-half hours at 110.degree. F. and then heated to 160.degree. F. at which time 2201 parts of caprolactam were added. The charge was maintained at 210.degree. F. to 220.degree. F. for about one and one-half hours until essentially all of the isocyanate moiety was consumed as indicated by the infrared scan. The batch was then thinned with methyl isobutyl ketone to 74% N.V.

EXAMPLE 1F

An acrylic anti-cratering agent was prepared by charging 44 parts of butyl acrylic, 15 parts of hydroxyethyl arcylic, 15 parts of dimethylaminoethyl methacrylic, 2 parts of styrene, 1 part of octyl mercaptan, 4 parts of VAZO 67, and 3 parts of acetone to a refluxing mixture of 13 parts of methyl isobutyl ketone and 2 parts of acetone over a four-hour period. After a 15 minute holding period, 0.145 parts of VAZO 67 and 1 part of methyl isobutyl ketone were added. The batch was maintained at the refluxing temperature for another hour.

EXAMPLE 1G-J

Emulsions were prepared by mixing 170 parts of the adduct of Example 1A, 6 parts of the anti-cratering agent of Example 1F, and 91 parts of the blocked cross-linking agents of Examples 1B, 1C, 1D and 1E, respectively, to 4 parts of acetic acid and 268 parts of deionized water under high agitation. After agitation for 5 days, the organic solvents were driven off. The following emulsions were prepared.

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     Emulsions                                                                 

              Cross-linker                                                     

                         Isocyanate Block Agent                                

     ______________________________________                                    

     1G       1B         TDI        ethylene glycol                            

                                    monopropyl ether                           

     1H       1C         TDI        ethylene glycol                            

                                    monohexyl ether                            

     1I       1D         IPDI       caprolactam                                

     1J       1E         TDI        caprolactam                                

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EXAMPLE 1K

An adduct 1K and an adduct 1L are the two intermediates for the grinding vehicle. The adduct 1K was prepared by charging one mole of ethylene glycol monopropyl ether to one mole of 2,4-toluene diisocyanate under agitation with a dry nitrogen blanket. The reaction was maintained at a temperature below 100.degree. F. The charge was held an additional one and one-half hours.

EXAMPLE 1L

In a suitable reactor vessel, 455 parts of Triton X-102.TM. (an alkylaryl polyether alcohol manufactured by Rohm and Haas, Philadelphia, Pa.) and 51 parts of methyl isobutyl ketone previously aqetroped to remove water, were added to 109 parts of 2,4 toluene diisocyanate. The reaction was maintained at 115.degree. F. for two hours. Then 56 parts of dimethyl ethanolamine were charged, and the reaction was maintained at 160.degree. F. for one hour. Finally, 50 parts of ethylene glycol monobutyl ether, 75 parts of lactic acid, and 89 parts of deionized water were charged, and the reaction was held at 190.degree. F. for one hour.

EXAMPLE 1M

A grinding vehicle was prepared by charging 88 parts of the adduct of Example 1K to a reaction vessel containing 206 parts of EPON 1002F (WPE=650 manufactured by Shell Chemical Co., Houston, Tex.) and 39 parts of methyl isobutyl ketone. The reaction temperature was maintained at 250.degree. F. for one hour. Then, 186 parts of ethylene glycol monobutyl ether, and 381 parts of adduct 1L, were added. The batch was maintained at 180.degree. F. for four hours.

EXAMPLE 1N

A pigment paste was prepared by grinding 191 parts of the grinding vehicle of Example 1M, 16 parts of ethylene glycol monobutyl ether, 370 parts of deionized water, 306 parts of clay, 64 parts of TiO.sub.2, 29 parts of lead silicate, 6 parts of carbon black, and 18 parts of dibutyl tin oxide in a steel ball mill for about 24 hours so that the average particle size of the mixture was about 12 microns.

EXAMPLE 2

Dispersions 2A, 2B, 2C and 2D suitable for electrodeposition baths were prepared comprising 539 parts of the emulsions of Examples 1G-1J respectively, 358 parts of deionized water, and 107 parts of the pigment paste of Example 1N by mixing in a suitable vessel with suitable mixing means. The dispersions had a P/B of 20/100, and total solids of 25%. After two weeks agitation, all organic solvent was driven off from the dispersions. The following dispersions were prepared.

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     Dispersion                                                                

               Emulsion Isocyanate  Blocking Agent                             

     ______________________________________                                    

     2A        1G       TDI         ethylene glycol                            

                                    monopropyl ether                           

     2B        1H       TDI         ethylene glycol                            

                                    monohexyl ether                            

     2C        1I       IPDI        caprolactam                                

     2D        1J       TDI         caprolactam                                

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EXAMPLE 3 Yellowing Test

One-half pint metal cans were electrocoated with the dispersions 2A-D, and the tops were sealed with aluminum foil to retain volatile components during the curing process which are believed to cause yellowing. The sealed cans were then baked at the temperatures indicated in the Table for 20 minutes, and allowed to cool. 4.times.12 inch steel panels were sprayed with a commercially available acrylic-melamine or melamine polyester white basecoat coating composition and a commercially available acrylic-melamine or melamine polyester clear coat coating composition, and were allowed a 15 minute flash. The aluminum foil was removed from the electrocoated cans as each can was inverted and placed immediately on one of the uncured 4.times.12 inch panels having the white basecoat and the clear topcoat. The cans and the panels were baked at a conventional topcoat system curing temperature for about 25 minutes. After the cans and the panels were removed from the oven and cooled, the cans were removed from the panels. The topcoat on each panel was visually observed for yelowing. The results are contained in the Table.

                TABLE                                                       

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                    Electrocoat Curing Temperatures                            

     Sample                                                                    

           Cross-linker   325.degree. F.                                       

                                  350.degree. F.                               

                                        375.degree. F.                         

                                              400.degree. F.                   

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     2A    TMP/TDI/ethylene                                                    

                          7       7     2     1                                

           glycol monopropyl                                                   

           ether                                                               

     2B    TMP/TDI/ethylene                                                    

                          8       8     3     3                                

           glycol monohexyl                                                    

           ether                                                               

     2C    TMP/IPDI/capro-                                                     

                          9       9     9     9                                

           lactam                                                              

     2D    TMP/TDI/capro- 8       7     3     2                                

           lactam                                                              

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      Rating System 10 = no yellowing 1 = severe yellowing (poorest results)

The use of the blocked polyisocyanates of the present invention as cross-linking agents in cathodic electrocoat primers in the process of the present invention, surprisingly and unexpectedly, eliminates the yellowing of a subsequently applied topcoat system. The method of the present invention results in topcoats having improved aesthetic appearance and durability. It is now possible to eliminate or make optional the costly and time consuming application of a primer/surfacer over an electrocoat to act as a barrier to separate and isolate an electrocoat from a topcoat.

Although this invention has been shown and described with respect to detailed embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail thereof may be made without departing from the spirit and scope of the claimed invention.

Claims

1. A method of preventing the yellowing of the outermost clear coating of a multicoat top coat coating system comprising

a. depositing onto a conductive substrate by cathodic electrodeposition a primer coating comprising at least one layer of an amine-epoxy resin adduct and a blocked cross-linking agent, and curing said primer coating to a hard durable film;
b. depositing a pigmented base coat/clear outer coat coating system as a top coat onto the primer coating by initially depositing at least one layer of a pigmented film-forming base coat onto the primer, said base coat comprising pigment and resin, said base coat having a conventional viscosity and said base coat further having a dry film thickness of about 0.7 to about 1.4 mils, and then depositing directly onto the pigmented basecoat at least one layer of a clear film-forming outer coat, said clear coat having a dry film thickness of about 1.2 to about 1.8 mils, and then curing the base coat and clear coat to form the base coat/clear coat top coat coating,

2. The method of claim 1 wherein the blocking agent is selected from the group consisting of alcohols, glycol ethers, amides, oximes and phenols.

3. A coated substrate coated by the method of claim 1.

Referenced Cited
U.S. Patent Documents
3984299 October 5, 1976 Jerabek
4038232 July 26, 1977 Bosso et al.
4246151 January 20, 1981 Dohy et al.
4345004 August 17, 1982 Miyata et al.
4375498 March 1, 1983 Le Minez et al.
4456507 June 26, 1984 Kivel et al.
4508767 April 2, 1985 Hokamura et al.
Patent History
Patent number: 4755418
Type: Grant
Filed: Oct 20, 1986
Date of Patent: Jul 5, 1988
Assignee: BASF Corporation (Clifton, NJ)
Inventors: Tapan K. DebRoy (Novi, MI), Ding-Yu Chung (Farmington Hills, MI)
Primary Examiner: John F. Niebling
Assistant Examiner: Ben C. Hsing
Attorney: Emil Richard Skula
Application Number: 6/921,559
Classifications
Current U.S. Class: Absolute Thicknesses Specified (428/215); 204/1811; 204/1817; Metal Base (427/409); Epoxy Or Polyepoxide Containing Coating (427/410); Of Epoxy Ether (428/413); Next To Metal (428/418)
International Classification: C25D 1306; B32B 702; B32B 2738; B32G 1508;